1. Field of the Invention
The present invention relates to an optical fiber and an optical transmission line using an optical fiber, more particularly relates to a reverse dispersion optical fiber having a low non-linearity suitable for wavelength division multiplex (WDM) transmission and an optical transmission line obtained by connecting such a reverse dispersion optical fiber and a positive dispersion optical fiber to reduce the total dispersion.
2. Description of the Related Art
With optical transmission being increased in speed and capacity, WDM transmission technology for transmitting a plurality of optical signals with different wavelengths multiplexed is in the spotlight as a key technology. In wavelength division multiplex transmission, however, the non-linearity in optical fiber due to the higher output of optical signals and to the interaction between the plurality of signal wavelengths has arisen as a new problem.
Among the non-linearity phenomena, the waveform distortion caused by self-phase modulation (SPM) or cross-phase modulation (XPM) is becoming a major problem.
Waveform distortion is inversely proportional to the effective core area (Aeff), so an increase in the effective core area has previously been sought in non-zero dispersion shift fibers (NZ-DSF). If the effective core area increases in a non-zero dispersion shift fiber, however, in general the bending loss or dispersion slope increase, so in practice the increase in the effective core area has been restricted.
Four-wave mixing (FWM), another non-linearity phenomenon, becomes remarkable when the dispersion is small, so an optical fiber with a small dispersion such as a non-zero dispersion shift fiber is disadvantageous in that point.
Therefore, to prevent four wave mixing in an optical transmission line by giving a certain large dispersion (wavelength dispersion) to the individual optical fibers while reducing the dispersion of the optical transmission line as a whole, comprised of a plurality of optical fibers connected together, close to zero, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-211511 proposes an optical transmission line having a reverse dispersion optical fiber (RFD) connected as a cable-type dispersion compensation fiber to a positive dispersion optical fiber. This is also called a “dispersion management optical transmission line”. In a dispersion management optical transmission line, a positive dispersion optical fiber having a small waveform distortion and a reverse dispersion optical fiber having a relatively large wavelength distortion and a reverse dispersion are connected at the front and rear, respectively. By introducing the signal into the front positive dispersion optical fiber to attenuate its power and then introducing it to the rear reverse dispersion optical fiber, the waveform distortion arising due to the non-linearity at the reverse dispersion optical fiber is made relatively small while reducing the total dispersion close to zero.
For example, the positive dispersion optical fiber and reverse dispersion optical fiber are given a large dispersion of about 16 to 22 ps/nm/km in terms of absolute value to suppress the four wave mixing. In that case, the dispersion slope of the positive dispersion optical fiber is compensated for by giving the reverse dispersion optical fiber a reverse dispersion slope. As a result, the positive dispersion optical fiber has an effective core area of about 70 to 130 μm2, while the reverse dispersion optical fiber has an effective core area of about 20 to 30 μm2, so there was a limit to reduction of the wavelength distortion caused by the non-linearity phenomena in the reverse dispersion optical fiber.
Recently, optical transmission lines emphasizing reduction of the waveform distortion over compensation of the dispersion slope have sometimes been demanded.